Simple prime number program - Weird issue with threads C#

Question

This is my code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;

namespace FirePrime
{
    class P         


        

Answer 1

The value the thread receives is the one startNum holds when the thread runs. To solve it copy the value into a local variable:

for (int i = 0; i < nrThreads; i++)
{
 var localStartNum = startNum; // save value in local variable
                                  // and use in the thread start
    var localIndex = i;

 var _thread = new System.Threading.Thread(() => 
                      MarkPrimes(localStartNum,
                                 Math.Min(localStartNum + interval, nrNums),
                                 localIndex));
 startNum = startNum + interval;
 _thread.IsBackground = true;
 _thread.Start();
}

Another bug in the code is waiting for all threads:

static bool AllThreadsFinished()
{
    bool allThreadsFinished = true; // Initialize to true instead of false
                                    // Otherwise, all ANDs will result false
    foreach (var threadFinished in ThreadsFinished)
    {
        allThreadsFinished = threadFinished;
    }

    return allThreadsFinished;
}

One tip which can help a little in synchronizing the threads: You can save all the threads in a list and join them from main thread.

var threads = new List<Thread>();

for (int i = 0; i < nrThreads; i++)
{
 var localStartNum = startNum; // save value in local variable
                                  // and use in the thread start
 var _thread = new System.Threading.Thread(() => 
                      MarkPrimes(localStartNum,
                                 Math.Min(localStartNum + interval, nrNums), i));
 startNum = startNum + interval;
 _thread.IsBackground = true;
 _thread.Start();
    threads.Add(_thread);
}

foreach(var thread in threads)
{
    thread.Join();
}